CHEMICAL AND PHYSICAL PROPERTIES, ETC. 209
good whiskey, but either, if not carefully used, can cause more evil than good. For this reason, guesswork in judging the amount of silicon an iron contains is not to be commended. Only by a knowledge of its chemical analysis can constant, uniform or desired results. in applying silicon to mixtures be best maintained. I have found that silicon had a softening effect up to about 4. oo per cent., or where it was possible to have castings jolted in safety over a pavement or rail track in transit for delivery.
This is as far as the founder ought to go in using such '£ poison '' to strength. After the carbon has become graphitic all it will, any further addition of silicon only closes the grain and makes the casting "soft rotten," or brittle. If, by still further addition we would exceed four per cent, of siliconówhich is a percentage no ordinary iron mixtures or casting requiring any strength at all should containówe may then harden the iron to a slight degree. A mixture having 3.75 per cent, of silicon is as high in that element as it is practical to use, if we expect general castings to hold together, unless the sulphur or manganese is very high to harden the iron. It is not desirable to have ferro-silicon iron in castings. Very' few general castings, excepting those for electrical purposes, require over three per cent, of silicon in their composition, if the sulphur or manganese is right, and the lower the silicon can practically be kept in most castings the better the results to be expected from its use.
In Russia, they have made light castings, as was shown in the exhibit at the World's Fair, 1893, with the silicon as low as .55, a little over one-half of one per cent., but in order to achieve this, we find the carbon in a fluid state, see Chapter LX, and the carbons, etc., in iron, as